Cycle problems?

Qu

Okay sooo i’m 3 months off depo & my period came back on in December & it lasted for 3 days . My second period came Jan 2nd & lasted for 6 days straight & went off on the 7th . Today is Jan 20th & i started spotting the 18th what could be the cause? 😩 My next period isn’t due until Jan 29th