In need of advice!

Hello I've just joined up to the site to get a bit of advice I had a baby just over 6 month ago I bled for 6 weeks then stopped and got my first period around the begining Of january this year I had unprotected sex on the 23rd of February I was due on on the 7th March but never came on till around the 17th or just after and inhave not had a period since then and I'm normally on time every month which is worrying me alot I've never ever had a problem just wondering if anyones been the same after having a baby or of theirs a possible chance o could be pregnant I've done a pregnancy test and couple of weeks ago but came up negative tia X