I was in the same suituation as you, i slept with my ex the day i finished my period and then every week after that i slept with my now boyfriend i was so worried, i just waited for my dateing scan and found out i was only a couple weeks pregs there for it is my new partners, i say just wait for your dateing scan to be sure, if you only had sex once after your ex and it was a week apart you will never know intill you get a dna. good luck its nerve racking