Sometimes I feel hurt by my mom and her behavior when I’m hanging out with my 73 year old friend what could it be?

Sometimes my mom seems to get so pissy with me when I stay the night at my old friends grandmas house she’s a 73 year old lady who I have known for about 2 years now she’s one of the sweetest older ladies I’ve ever met and sometimes to take a break from work which I dog sit whenever I get the chance to have a break from it I’ll stay the night with her and keep her company watching movies with her all night and enjoying our talks. She loves me like her own and takes care of me as such. We hang out and have girl days when we just go shop and eat out together and I’ll take her to doctors appointments and take her out for a late night ice cream. I’ve always loved my mom and am so close to her and we hang out mostly daily. But sometimes she’ll be at work and I’ll go to my Grammys (this is what I call her) house. My mom will call on her way home from work and catch attitude with me if I bring up that I’m with her or going over there. It hurts me because she won’t speak to me about what’s wrong what’s hurting her I ask her if she’s jealous or why she acts the way she does but she never responds and never gives a direct answer and will say she’s fine. It hurts my feelings and I’ve expressed this to her many times and she never wants to talk. She doesn’t have any friends and refuses to have any and seems pretty bitter towards people but loves my grandmas best friends and will hang out with them. They are in their 80s. I have no friends other than my Grammy and mom and always make time for both since I love them we also go to Bible study together. I don’t understand does anyone think they could maybe know why my mom behaves the way she does? It hurts me so bad and she always says I’m sorry if you feel that way. But it’s the attitude and the fact she acts like I’m never hanging out with her whenever I always do.

Sometimes I feel hurt by my mom and her behavior when I’m hanging out with my 73 year old friend what could it be?
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