Help!

So I’ve been married since about a year and a half I never have done anything with anybody else besides my husband. When we first got married we barley had any problems in our sex lifes we would always have fun and he would love it and could go on for hours •I only once had an orgasam but that’s another story!• recently our sex life has went down the drain we barely do anything anymore I mean come on we’re 22 and 23 years old we’re supposed to be crazy! Cut a long story short every time we try to have sex it will be great at first but once he puts me on top it goes down hill my legs always shake not in a good way in a pain way recently I can only be on top for 2 min’s top and then I have to stop from the nevers in my leg has this happen to anybody else ? Or just me ? Any tips anything would be nice ! Please!