Im soooo confused bout how far along i am??!!?

Back in April i had a BLOOD pregnancy test done... it came back negative. I'm now pregnant. I found out around june 24th when i started to feel n suspect something. so i got a urine n blood test done...both were positive. I figured i was 11/12 weeks along. Around May 26th( my last period) i had a light spotty period. I went a couple weeks ago to have an ultrasound done. the GA said 11w4days. But the tech said i was 20/21 weeks along. This will be my first child. Im very slender n barely showing. i dont feel as if im 20something weeks. Could that blood test have been wrong?? Im confused!! Help!