What Do Small Red Spots On Arms Indicate?

Hi, I have been getting - on infrequent occassions - small red spots on my arms after sleeping naked with my arm under my girlfriend. The spots look slightly similar to heat spots, however we ve slept both in her room (which is ridiculously cold) and mine (which is pretty much normal room temperature most of the year) and I ve sometimes got them regardless of where we ve slept. The spots go after 5-6 days, and fade after the first 24 hours, but are extremely itchy. I wasn t sure if it was due to sweat when sleeping, a reaction to her skin (which sounded a bit over the top) but we really don t know and they itch quite badly sometimes. There are normally a large amount around the bicep area, with a few towards the forearm region. Any help at all is greatly appreciated as it s been a concern for a while but tricky to see the doctor over as I work 7 days a week.