False positive test Lyme?

About 2 weeks ago I developed a weird rash overnight that looked like hives. I had never had it before so I went to the doctor who did blood work. The rash went away 2 days later. A couple days after they said a preliminary test came back positive for lyme and when that happens the lab does a 2nd test which also came back positive for lyme. Well... about a week and a half later they wanted to do more blood work to recheck it and I just got a call saying it was negative! I'm very happy but why did this happen? Why would it come back positive the first time and negative this time? I'm going to be rechecked in 4-6 weeks.