This letter informs your partner that there's something about him or her that is making you allergic, and you need time to find out what it is.
I think we've got a bit of a problem. Ever since we started seeing each other, I've been having problems with sneezing and watery eyes and all sorts of things. At first I just thought it was your <potential cause>, but I don't think that's it. The only conclusion I can come to is that I'm allergic to you. I know that sounds bizarre. But what else could it be? I think it would be best if we didn't see each other for a while, until I have a chance to find a cure. I hope you can understand that I'm just trying to do what's best for both of us. I'll call you when I've made some progress. Until then, thank you for everything.