Guys help!!

Star
I was due my period on 20/3/14, it is now 25th and including the 20th I'm 6 days late! I'm never late either I'm so regular it's unreal! 
​I took a test yesterday in the afternoon and got a BFN 
​Visited my doc yesterday and he told me to do a tribe sample! Did it this morning with first pee of the day and sent it to the docs and results came back another BFN! 
​Doc told me over the phone to wait 7-10 days and if AF still hang arrived then test again! 
​I can't afford anymore tests so my mum is going to get me some at the weekend. 
​Can I still be pregnant?
​Has anyone ever been this late before and git lots of BFN's but still been pregnant? 
​When do you all suggest I take another test? Argh it's killing me!