So, my partner and I broke up in late February. I had been on the pill. After we broke up I realized that My period was two days late but had a negative test. I had a one night stand with someone else. This person pulled out. 16 days after this I found out I'm pregnant. I'm sure that it is my ex partners child due to the fact that I was already late when I had sex and that if it had been the one night stands child I couldn't have gotten a positive test back in 16 days. But now the baby is measuring smaller than it should be. Could it possibly be the one night stands baby?
It is unlikely to be "someone else's" baby.
You would have been at/near your lowest fertility having sex when your period is due. Basically from about 10 days prior to your period start date to 6 days afterwards you are NOT likely to get pregnant. Before your period is due the egg is not viable. You won't then ovulate until about day 11 of your cycle at the earliest. Sperm can survive up to about 5 days max.
It's your ex's from your description. A DNA test can confirm.
Disregard the measurements and estimated due dates. They are all just estimates. Fetuses, like people, develop at different rates/times.