could be due to a few different things

- lubrication. especially if condoms are being used. KY or something like that could help.
- allergy. weird... but some women can be allergic to the sperm of a particular guy.
- infection. thrush etc. a doc could rule this one out...

if i haven't had sex in a while then when i start i think the alteration throws out my ph balance or something... takes my body a bit to adjust...