I found this information about the HCG shot:
"You should wait 7-14 days after your last injection before the shot is out of your system. If you test too soon, you might get a false positive."

You may need to skip a shot or two to make sure you get an accurate reading on your pregnancy test.

HCG is the hormone that pregnancy tests are reacting to. It's a good bet that the shots you receive are what the test is reading. I would make sure the doctor knows you are taking the hormone shot and s/he can do other tests for pregnancy.

i was taking hcg for about a week when i missed my 1st period i took 4 preg test all of which were possitive. i had my mother in law take one too because she was on the hcg as well, hers was negitive. i went to the doctor and my urine test was negitive. now i've missed my second period and still negitive pregnancy test. im getting headaches all the time and constantly tired even though i've been off the hcg for over a month. i dont know if the hcg stopped my period all together or if i am pregnant. i guess i'll have to wait till missed period #3. does this happen often?