Nickel allergies

Yes it's the same with full bracelets. Though it may vary from watch to watch as you noticed.

For me it also happens after long wear. A week or two I think. And tightness and temperature might make a difference too. 

Titanium is the way to go. A lightweight watch worn super loose with a band that remains circular (not hard plastic/rubber that tries to be straight) is another option and is anyways very comfy if the watch doesn't have pokey edges.

Not slightly loose. That prevents wrist motion. Gotta go a few steps looser. 

G Shocks work well super loose like this because they keep a good circle. Especially the sporty ones with the slightly softer material. 

It seems to be the combination of prolonged contact and sweat that let the nickel leech out and cause a reaction. Once your skin gets sensitised your immune response will stay in a heightened state for a while. Thus even watches that didn’t cause a problem before will do so now.

Often if you remove any cause of irritation for a while (depends on person, maybe 4 weeks), the response will have died down so you can wear the watch again.

Best to wear looser and limit  length of time on wrist.

Titanium is a great way to go, no nickel content.

Or a suitable nato strap.

Short term to help irritation can be use of a mild steroid cream (though not recommended as a long term strategy, max 7 days maybe and not to be repeated for a long time).

Dermatology time!! An adverse raction to Nickel is unfortunate and will mean you won't be able to wear anything that has nickel content in the caseback, bracelet or strap furniture. A real shame, but them's da breaks. Titanium is a great option if you fall into this boat. 

That said, it's quite uncommon to suffer a specific metal allergy in skin - horrible if you do, but uncommon. 

More likely is the combined effect of prolonged wear keeping an impermeable surface (the metal) in close contact with the skin - that part of the skin does not dry completely, and the watch/bracelet compresses it ever so slightly, reducing the effectiveness of blood flow to remove fluids. Both add up to increased susceptibility to surface irritation, primarily from bacteria, but also from metals and other contaminants, even for those who do not have an allergy or intolerance. 

Contact points with metal are always more susceptible to irritation than plastic, rubber or leather, as those materials (to some degree) absorb or wick away contaminants in the skin. Metal does not. 

In short, the contact points with your watch are a cesspool of surface bacteria, and wearing the watch constantly conditions the skin into the worst possible state to become irritated by it (short of actually cutting the skin!). You can get the same issue with leather or rubber straps - less so, as they are more effective at absorbing contaminants than metals. 

Before abandoning all hope - if you do suffer, more regular cleaning of the watch parts, and taking a day out now and then to let skin recover, is a great approach. Likewise, using a non-steroidal, lanolin-heavy moisturiser will help the skin recover it's condition and blood flow. 

I have a nickel allergy, or reaction some kind of metal. I've had pretty serious rashes from belt buckles, glasses, and I can't wear a chain around my neck (or at this point, it's not worth even trying). I haven't reacted to watch buckles or bracelets though. I have had minor irritation under the head of the watch, but I think that has more to do with heat and sweat, and it goes away.

It's easy enough to swap the buckle on a strap. They're generally held on by a spring bar. There's often even a handy little hole, like drilled lugs. Generic after market buckles are readily available, so you could try to find one that works for you. Or if you have a strap that you don't react to, you could swap out the buckle to another strap (yes, I've worn misbranded buckles, like Seiko on a Nomos; I like the deployant). You just have to make sure the buckle is the correct width.

Also, I've used clear nail polish on the offending metal, which sometimes solves the problem, at least temporarily. Better than cling wrap. 

Thank you for the good tips!