What Causes Brown Spotting And Mild Nausea Inspite Of Negative UPT?

My period was a week late and when it did come, it was just a pinkish Brown spotting for about three days. After my husband and I had sex, there was also pinkish Brown spotting (has never happened before). For the past three days I ve felt mildly nauseous all day (even after eating... Also feels constantly hungry). Have taken two pregnancy tests and they are negative. The timing seems weird, but thoughts on if I might be pregnant, despite the tests?