Yesterday BFP and today BFN

Nicola • I`m a 32 year old teacher from the UK. Had my rainbow baby in September 2017 after miscarrying twice. Had a chemical pregnancy in January and now TTC #2
I'm so confused, yesterday I did a test and had a BFP. I've done two this morning and had BFN 😥 Any suggestions? I wanted to surprise my boyfriend today...Â