Pregnancy scare

I had unprotected sex on the 5th of September and the 7th. A week later I had spotting that lasted about a week. I had mild cramps and the bleeding was very light. Usually when I have a period I have severe cramps and heavily flow. I calculated that my ovulation day was on the 5th of September. I have taken three pregnancy tests that came out negative. The first test I took on the 14th, the second one I took on the 19th and then the third one on the 21st. I have recently been experiencing pregnancy symptoms such as frequent urination, breast tenderness, lower back pain, fatigue, feeling sick, gas, insomnia, constipation, diarrhoea and stomach cramps that appear on one side. As each day goes by I get more Symptoms.I am also 14 days late for my period so I am not sure what to do as the last few tests have come out negative. Could I be pregnant or is it just my body acting strangely?