Implantation bleeding? (TMI)

I just need some advice...

I started bleeding bright red blood 4 days ago, light enough to have two pant liners in a day but no more for the entire time, always bright red in colour.

My last period was 22nd March. Spoken to my GP and she said to test but a digital was negative today and a ‘6 days early’ test two days ago was too.

What’s happening? How long does implantation bleeding last? Is it that or something bad? GP didn’t seem worried but I’ve read about polyps etc.