Back pain and vomiting. Be the reason?

Hi,Me and my partner had a sex on 05/08/2022 and because of contraceptive failure (condom tore up) it turned up into an unprotected sex. However not sure semen has entered or not. But still she took a pill ie., I pill on the same day within 12 hours. The day when we had sex was 6th day of her periods. Her periods began on 31/07/2022.She had bleeding in clumps fomr 12/08/22 to 18/08/22. She has a 30 days menstrual cycle normally. After that we had pregnancy test on 23/08/2022 and it came up negative.Also we had consecutive pregnancy tests on 1/09/2022 and 4/09/2022 and 09/09/2022 which all came up as negative.On 11/09/22 she started bleeding again slowly and by 12/09/22 it came up to a normal flow. This bleeding stopped on 18/09/22. I believe she didi not skipped her periods. Now she is telling she is experiencing lower back pain feeling like vomiting. What could be the reason?