What Causes A Weird Odour From The Vagina?

a few months ago I had a strange odour coming from my vagina ; I had spoken to the doctor and had tests done for sti s, and the results were clear. The smell lasted for months and eventually disappeared. The doctor and I were both unsure why this was. At the time had started taking cerazette again for the first time in 9 months, which I have taking since I was about 16 and I ve had no problems. This time, I had been bleeding continuously after about two weeks from taking it - this being the reason that I decided to stop taking it and the bleeding stopped almost immediately. About 3 weeks ago I started taking this pill again, and again after about 2 weeks I began bleeding continuously again but what has bothered me more is the fishy smell which has returned, and is the same as what I had before, a few months back when taking cerazette. I m now assuming this is related to taking the pill and again I ve stopped taking it. It had taken months for the smell to disappear before. I ve not taken it for as long this time but can you please indicate how long it will take for this smell to go back to normal, and why it is happening? And do you think I would need to see a doctor to discuss this?