Paternity questions after using AncestryDNA

Ever since I took my AncestryDNA test, I've had some lingering questions about my biological father's side of the family tree. My mother took the test and there has been no issue matching her people to the people written in our family tree.

However, when I look at the DNA matches from my father's side, the surnames do not match to any name in his family tree, which has also been well-documented.

I have a brother who is most certainly my father's son, based on shared characteristics and voices. He took a DNA test last month and are waiting on the results. My father passes years ago and my mother and I do not have a close relationship. We know my mother cheated on our father when we were both children, but are unaware of any cheating before we were born. Without evidence, I feel uncomfortable asking if my father is my biological father.

Today I logged into AncestryDNA and noticed a new match, the closest match I've had (after my mother) that has been designated "Close family-1st cousins". I've messaged the user but have not yet heard anything. I understand "close family" could also mean aunt/uncles, nieces/nephews. The match's name is not familiar to me and it is not shared with my mother.

Does anyone have any insight on this? I am not aware of any philandering on my father's side. They are Irish Catholic, were married until my grandfather died and had 7 kids altogether.

I'm really freaked out right now and am worried my father isn't who my mother said he is.