Feeling told wrong gender at 20 weeks

Shams
Hello everyone,
This might sound silly but anyone feel they were told the wrong gender at their 20 week scan. Long story short, my husband and I decided we didn't want to know gender, but as I was at the appt by myself I was very tempted to find out. The ultrasound tech gave me an envelope and I peeked from one side and saw blue..it's a b.... I then quickly threw away the envelope. For some reason I have this niggling feeling it is wrong and I don't know if it's more wishful thinking (I have 2 boys already) or because I didn't look at the envelope properly.  I keep reading posts where the girl genatalia is swollen and can be mistaken for a boy and this isn't helping me either! 
Anyone else feel they were told wrong gender? 
Dh