Positive test last night, but negative test this morning/afternoon?!

Ka
Hi all.. I had a clear positive pregnancy test last night, faint, but the line was there and was pink. This morning/afternoon I took a test and it came out negative. I'm about 11dpo today and I used a First response test both times. 
Has this ever happened to anyone else and still ended up being pregnant after all? I don't understand why I got a positive at night but not the next day. Thanks in advance.. I'm so upset..