Re: abdominal pressure during sex..? due to presence of hymen??
The abdominal pressure sounds like his penis might be pushing against your cervix, especially if you feel it mostly when you're on top. So you'll have to be more careful when on top if you want to avoid that. Tell your boyfriend to be careful too. If you're both unable to work around it by only penetrating part ways, then simply don't have sex with you on top.
The pain you feel during initial penetration is probably your hymen being ripped or stretched again after having healed since the time before. After a while it should stop happening, but if it keeps being an issue after a month of active sex life you should probably see your doctor about having it surgically removed so it won't bother you so much.