Confusing blood tests!!??

Kind of a long story, sorry! My last cycle started June 16th. On July 20th I got a faint positive on a hpt. I called my regular doctor the following day and went in. I also got a faint positive on a urine test at their office so they sent me for a blood test. She told me the blood test showed I was only 4-5 days along (reasons for the faint positive). Judging by my cycle I should have been approximately 4 weeks she said but I wasn't. They had me do another blood test the following week and it showed I should be about 4 weeks so she she had me take a third blood test the following week (was this Thursday). Yesterday they called and said my pregnancy is progressing nicely but my HCG levels this test corresponded more with my first and showed me to be about 3 and 1/2 weeks along. She said it all looked fine and I could contact and OB to continue my appointments with. Anyone else have this kind of situation? I know it means I got pregnant at a weird time during my cycle but the differing HCG levels during my blood tests kind of made me nervous!! Thoughts from mama's who have been through or are also pregnant? Thanks Ladies!!!