Am I over reacting in this situation with my mom?

My mom is like my best friend. And she’s a great grandma to my daughter. Anyways, she had met my latest step dad right before I got pregnant with my daughter in 2013. That is my daughters grandpa as far as she’s concerned because my dad really doesn’t have anything to do with me. They separated in January of this year and around that time she started seeing someone from work. I’ve only met this guy one time and my daughter has met him very briefly two times. I’m trying to kind of keep her away until I get to know him. Because I don’t want her to get to know someone and then boom he’s gone. And I will say this guy seems really nice the one time I met him. Anyway. My daughter used to stay every weekend at my moms house but this summer she hasn’t been wanting to go over there which is fine, she’s just been clinging to me. So my daughter called my mom the other day telling her she misses her and my mom came up with the idea that she would come pick her up on Sundays and take her somewhere and just spend time with her. Here comes my problem:

My mom texted me Friday saying she was taking her to a water park or the beach on Sunday. I said I was fine with that. I forgot my mom doesn’t really like water and I told her I couldn’t go because I have a lot of stuff to do with school so everything was fine and dandy. She said she’d come get her around 8:30 and take off. So last night she texted me saying that since she doesn’t go down the water slides she invited her boyfriend so he could go down the water slides with my daughter. I’m uncomfortable with that and I told her that I would rather get to know this man a little more before my daughter goes out of town with them. I guess she took this as I don’t trust her with my daughter and she started arguing with me about it. Basically saying that I was ruining my daughters day and that I was being ridiculous about feeling uncomfortable with her “friend” being there and that I let my daughter be around people all the time. Which is not true, I’m pretty picky with how is around my daughter. She barely even gets to see her other grandma because she is a wreck and a half but that’s beside the point.

Sorry this is so long but I just need to know, am I being ridiculous? Honestly I probably would have let her go because I do trust my mom but her dad was also not comfortable with her going and said no so. If I’m wrong I want to apologize to her but I honestly don’t feel like I’m being ridiculous.