pregnancy questions

GF
I just have a few questions. 
I've taken 3 home pregnancy tests, all positive. 
Is there anyway that they are wrong? (I hope not) 
I have missed my period ever since march 
But I don't remember my last exact period date for march. Only February. 
At my doctors appointment is it a problem to not know my very last period date?
Also, I'm very small 109 lbs. I am about maybe 3 months pregnant.
Should I be showing? When will I possibly show?