You had sex on day 5 of your cycle.
You then had sex on day 7 of your cycle and after with your ex.
You likely ovulated between April 30 and May 5, days 11-16 of your cycle. Sperm can survive up to about 5 days max.
It is unlikely that the one night stand guy is the father. Timing wise it was early. You had sex with your ex at/near your peak fertility. He is the likely father.
Your due date corresponds with an LMP starting on April 24. It is however just an ESTIMATE. I'm basing dates on your KNOWN LMP. Your doctor CAN NOT tell when you conceived or when you ovulated - after the fact.
A DNA test can confirm. Good luck.