My wife took Vitamin C tablets after a negative pregnancy test and has back pain. Is it related?

Hi doctor,

The period cycle of my wife is usually 30 to 32 days. The last period was before two months and 21 days. And, we had unprotected sex on her sixth day of periods. After 26 days in the same month, she got her periods. She got her periods in the same month we had sex. But, in the next month, she did not get her periods. We did a home pregnancy test, and it was negative. We did it with two different brands early in the morning, and the results were negative. We did three to four times after two weeks of missed period. Since the results were negative, she took tablet Vitamin C chewable 2000 mg daily for three days. She never had Vitamin C before this. The next day she started having left lower back pain with mild nausea. Is this due to Vitamin C? She has a history of kidney stones. But now she is alright. Should I be worried? Even after getting the periods the month, we had sex and a negative home pregnancy test, can she still be pregnant? Please help me with your answer.