35 weeks pregnant and belly goes hard, uncomfortable and a lot of pressure after sex is this normal?

Im 19 years old and 35 weeks pregnant today with my first baby, the past few weeks anytime i have sex my whole stomach goes hard like a rock and i feel a lot of pressure that its sore and very uncomfortable i also get bad cramps in my legs if i stand up or walk around so i have to lye down . Is this normal? it turns me off having sex. It lasts about 40 minutes then goes back to normal and i can feel baby moving again. Now i can understand that sex can help you go into labour, is this true? has it worked for anyone?