Can Anyone relate??

I can get pregnant quickly! As both attempts my husband and I got pregnant that month of trying! But on both of our attempts, we are not seeing a sac in the uterus?? 1st pregnancy, Levels were at about 4500 about 6 weeks. So it was low, but we couldnt see anything on the ultrasound, nor in my tubes? Second pregnancy, levels got up too 361, but when down from there. So we are wondering if egg is getting fertilized, but I'm having trouble with implantation in the uterus? Anyone with similar experiences?? Please share! As I dont think my tubes are blocked, as I am able to get pregnant, plus I cant afford an hsg test right now! I'm 27, husband is 30. No known health issues on either side!