does size = pleasure?

So over the past year sex was amazing with my boyfriend. Never had any problems , but recently I've noticed it's not as great as it use to be. My boyfriend was my first "real" sex partner. When we first dated, sex was painful and not very enjoyable. Now I feel like my vagina is stretching and sometimes I have a hard time enjoying sex (sometimes I don't even feel like I'm being penetrated) . I love him but I just need to vent. some advice would be nice also. Sex isn't always bad but it's not always good. Ladies, is this normal?