Positive Tests / HCG really low

I have been getting pregnancy positive tests (faint line) but went to the doctor and the hcg level was low 16.

I’ve done a pregnancy test again and it’s again a very faint line.

I have to go for another test in 2 days, but has this happened to anyone else?

For the last 6 days it’s been very faint. Periods are due in 5 days :(