Can I be pregnant?

I had my period over 5 weeks ago. 3 days later I started bleeding but wasn't like a normal period ( 4 nearly a week) never happened before! Today I woke up with bad back ache, I took a test last week and 1 said negative and the other was invalid! Ain't taken one since! Anyone experienced this before? I know no one can answer am I pregnant or not, until I re test! Just wanted 2 see how many people have had this happen! I'm also a mum of 2 and never experienced anything like this x