I've Had The Nexplanon For About 5 Months, I Had

I've had the nexplanon for about 5 months, I had 'unprotected' sex, and then 18 days later I took a clear blue pregnancy test and it came out negative. But I've had horrible cramps and really dark heavier spotting the past few days, even though I haven't had my period since I got the implant put in. Could I be pregnant (even though the test came out negative) or is this just a side effect of the implant? Please help, thank you.