Am I ungrateful?

I’m a single mum to a little boy I have no help financially and I struggle to make ends meet and that’s with a job and gov support, we don’t get much, my son has been asking for a ride along truck for months and it cost £219 so I’ve been telling him to wait until his birthday and maybe he’ll get it when realistically I’m saving every penny I can to be able to buy it, his birthday is at the end of the month and I mustered up the money for it last week and went ahead and bought it, I have been saving for this for about 5 months and was really proud of myself, my mum knows about this and knows I’ve been saving every penny I have for it, so my son stayed over at the weekend and he come home Sunday night with the brand new truck. She had went and bought it for him and he’d been playing on it all weekend. I broke down crying saying she knew I had just bought him it and that he was getting it in a few weeks on his birthday and she says to him, not me, him “don’t worry darling granny will get you all the good toys” and told me to stop being ungrateful and accept the gift, she knew I had bought it, she was WITH ME when I did, and she knew he would be getting it on his bday because I couldn’t afford to give him it early and then buy another big birthday thing, but she said he deserves it early so went and got him one herself. She won’t loan me money to buy things for him, I had asked and said I’d pay her back over a few months but she says that’s never going to happen and I’m an adult. I told her to take it home and keep it at her house, that way he can have one here and one there and she refused saying she doesn’t have room for it and to just return mine and stop being a brat. I’m 26 for reference. It’s not about returning it, it’s the concept that she knew I’ve been saving like hell and I felt amazing I had managed to save enough for it and she went ahead and bought it after me to give him early because “he deserves it” and the way she said to him she’ll get the good stuff made me feel like shit like I can’t get him anything good. Am I just being ungrateful? Can’t stop crying