Its totally normal not to feel baby all the time. Babies have rest periods like we do. Only we have night and day to tell us when to rest, whereas babies just go, go, go until they tire themselves out
Your still pretty early ro be having consistant movement. some women don't even feel their baby until like 24 or 25 weeks. i wouldn't be too concerned. your baby still has alot of room to move around and "hide" from you. and yes it is safe to have sex unless your dr. Has advised you against it. enjoy.
It's different for everyone. I have a girlfriend, it was her first pregnancy as well and see didn't feel her little girl till 24 weeks. Another friend first as well. 23. I am VERY fortunate my little man decided wanted to start early. I've felt kicks since 18 weeks. 20 weeks yesterday and they get stronger and stronger every day. Some take my breath away