What Causes Scanty Period And Nausea After Discontinuing Depo Injection?

Hi I had my first depo injection in April and was due for it again in June but never went for it I didnt have a period whilst on the depo and havnt seen anything whilst missin the depo but a few days ago I had what could be a period but I m not sure it started off red for a day then turned to a brown then dark brown black lasted about 4 days and I have recently been feeling very sick and light headed all the time and having a lot of headaches can u please tell me if this is normal or could I be pregnant as me and my partner are having unprotected sex