I am pregnant?

IJust wonder if anyone can help? My periods are normally exactly on time never late never an issue My last period started the 1st of April for 4 days in that time I have had a lot of sex with my boyfriend I was due on my period the 28th of April but still aint come yet I did a urine test at clinic on 2nd of May it was negative Then on 4th may I became really light headed and felt sick went to hospital they did a urine and blood test both negative then late sat night and sun morning I had a little bit of spotting it was a pinky brown colour an only a very tiny amount The Dr said that could be a sign of implantation and it takes on average 9 days to implant after fertilisation which would make my ovulation roughly around the 25th of April Which makes sense due to the clear really slippery discharge I had just before then How soon after what may be implantation bleeding could I get a positive test?? Normally my cycle is exactly 28 days never late or just a day never a week! What are people's thoughts Could I be pregnant? And when should I test??