Implantation or period?

I’ve been tracking my period for the last 2 months with eve also the ovulation days last month had sex about 3 days before ovulation according to this app after being on the pill for a little over a month and on the nexplanon about a little over a month before that. Before bc my periods were never regular and this app never predicted them correctly. So fast forward I’ve been having this dull lower back pain, headache, constipation, ever since 3 dpo and at exactly 9 dpo began to bleed even though my period is due the 5th. So now, the period started light but it usually starts heavily. I’ve read implantation bleeding shouldnt be heavy but it’s starting to resemble a period and doesn’t look like it’s going to end soon. Today is my second day bleeding. Now my question is should I trust this app and assume it’s implantation or just my period? Btw took a first response today and got a negative. Anyone been through this or have any advice?