Single mom GF did not tell me she's going out with the father of the kids and I felt uncomfortable about it, is it an issue?

We been dating for almost 3 years now and supporting her financially since day 1. I just stopped for 3 months now since my income has gotten low. She told me she's going out to buy things for her kids. I put the pieces of the puzzle together and realized she going out with the father of her kids. She is still communicating with her and sometimes stay at their home for few hours without me there. Is there an issue with her hiding it from me that she is going out with her ex? She just told me now when I asked her. This is for schooling and says there is no issue but I still feel uncomfortable. That's her ex.