Frozen Emryo Transfer

Hi, I am sorry for the English, but I live abroad with no Hebre keyboard. We have been trying to get pregnant for almost 3 years. We went through few IUI AND did not get pregnant even though nothing seemed wrong. I then did laporoscopy AND was diagnosed with Endometriosis. I was treated during the surgey. We tried again few IUI AND then did one cycle of IVF last year. I got pregnant but misscarried on week 8 (the embryo stopped developing in after week 5). We did another transfer of frozen embryo AND will be doing another one in few days. Is it true that it is recommended to have sex before AND after the transfer in a frozen cycle? How close after the transfer is it safe? Is it also true that there are some "liquids" in the uterus that are considered bad for implantation? How do you diagnose that? They never do for me any blood tests after the transfer until the pregnancy test - how can I know that I receive enough progestrone (I take 1 crinone a day). Many thanks for you help.