No babe. There is NO WAY POSSIBLE to get pregnant before your last period. It would be a miracle if that were the case. The sex you had in April is the one where your baby was conceived, not before your period, not the 31st. Your baby is 2 weeks old, hence your 2 - 3 week result on the clearblue.
Doctors add on these extra weeks as they have no clue when you ovulated and will never sit and do the calculations for you.
I really hope I've been clearer hun, I know your head must be mush at the minute trying to process everything. But the important thing is, you are pregnant. You need to stay calm and see your doctor so you are doing the best for your baby.
I wish you luck x