Brown fluid discarge. Or any test!

My wife is 20 years old and has noticed brown vaginal discharge/spotting for the first time. Her last period was approximately 15 days ago, whereas she usually notices any brown discharge 4–5 days after her period. This time it appeared around 15 days later. The discharge is minor and there is no unusual/foul smell. She has also mentioned that she does not drink enough water. A few days before this, during sexual activity, there was fairly vigorous external rubbing/fingering while she was still wearing jeans. There was no direct fingering inside the vagina underneath the clothing. We are worried whether the vigorous rubbing could have caused irritation or minor injury, or whether the brown discharge could simply be old blood or spotting for another reason. She has never experienced this before and is quite anxious about it. Could you please advise what the most likely causes are, whether this could be related to the sexual activity, and whether she needs an examination or any test