What Causes Rash Around A Contraceptive Implant?

hi i have the contraceptive implant (inserted 1 and a half years ago) and i have never had any problems with it, but now i have just noticed a bum and a rash around it (kind of looks like a nettle rash or an allergic reaction) will the implant still be working properly as i had sex a few days ago and I m scared it won t have worked?