I have been taking my pill regularly and on time daily.
I was due my period last Thursday, today is Tuesday and I still haven't gotten it.
I took a test on Thursday and it was negative.
Could there be another reason for missing my period??

You should take another test maybe on Friday. Be sure to use first morning urine as it contains the highest concentration of pregnancy hormone (hcg).

If then it is negative you should schedule an appointment with your doctor for a blood test.

Ive just taken another test at lunch time... a quick response one. It also was negative. I suppose all I can do is wait until Friday again. But will it matter that I'm starting back on my pill in the morning?

You have to take your tesst first thing in the morning for it to be as accurate as possible.

Please read my answer above.