It can certainly be because this is the first time in a while. Sex is actually quite traumatic to your general and very small micro tears are common, even with regular sex.
Make sure you are WET
Take it slow
Spend some time on foreplay. Just because you are wet does not mean you are ready.
Guide his penis in and make sure he thrusts straight.
If this is only the second or third time you are having sex, your hymen can still cause bleeding.
If it keeps on happening, you might want to see your doctor.