unless your water has broke, you are at risk for preterm labor, have been bleeding, have placenta previa, then sex is safe. the achey pain is pretty normal so is having contractions after orgasm. the baby is safe and can only feel a rocking sensation, he or she is well incased in fluid and even if the baby has dropped it will not hurt him, but will help lead to your discomfort.
And the baby can be head down and still not have dropped yet.
Sex can still be enjoyable for you if you have your bf take it slow and let him know if he is making you uncomfortable. and knowing you aren't hurting your baby should ease some of your fears so that you can enjoy it more..
Hope I have helped a little at least.....Good luck with everything and if you have anymore questions feel free to post again.......