How is this possible?

So back in March-April I found out that both of my fallopian tubes are blocked. The right one completely and the left one only down at the end. They told me it was not possible for me to get pregnant without something like IVF. I found this out because I had 2 miscarriages and an ectopic pregnancy.

Today I am a week late on my period. I figured it was probably normal for the surgery I had (laparoscopic surgery to look at my tubes and uterus and stuff). Just to ease my mind though, I bought 2 pregnancy tests. I figured it would be negative. WRONG. It was a VERY bright positive. Showed up in less than a minute.

I dont understand how this is possible when I was told there was NO WAY I could conceive naturally. Please someone help explain this!