Lower Back Pain. Could 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.  On 12/08/2022 she had pain below her stomach and waist. She had vaginal Bleeding also. Though not so much but very less. It was dark brown in colour and this bleeding stopped on 17/08/2022.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. Now she is telling she is experiencing lower back pain feeling like vomiting. What could be the reason? Kindly help.