OMG I could die 😰

My baby is three months old and has been exclusively breastfeeding since birth. I’ve been at work for about a month now , and he’s been going to my grandma house or my moms house . I pumped a rather large stash before I returned to work and still pump now . I usually pack my baby three to four frozen bags of milk a day when he is away from me , so that I don’t thaw out more than needed. SO TODAY I packed three and my son used them the first part of the day , tell me whyyyyy my grandma went to the store and bought READY MADE ENFAMIL FORMULA!!!! And gave it to my son!!! What makes it so bad I called and asked how he was and she said okay , all she had to do was call me , I have a freezer full of milk!!!!!! Instead , her being a grandma she goes and purchases F O R M U L A!!!!!!!! I’m livid and feel like a bad mom!!!! Like my baby only knows breast milk I’ve never ever ever mentioned formula why would u give it to mu baby!!!!!!! I’m so upset crying real tears man!!!!!!