91 days since last period!

Last period 17th January 2017 -
91 days ago!
Negative home pregnancy tests (1 per week since being late!)
Negative doctors blood test (taken at 45 days since last period)
I'm normally regular, with cycles every 30 -32 days. Periods last about 5 days.
I have another GP appointment at the end of the month, but doctors seem dismissive, and pass of as 'just one of those things'. I don't understand why my periods have just stopped!
Has anyone else been this late to start their period? Any advice on how to get NHS GP to follow up with hospital referral/ investigations etc? 
Many thanks,
Louise