Could it be seperation anxiety at 5mths?

My daughter went through this at a similar age. We dealt with it by Me going "bye" every time my mum was over - even if it was only out to hang out the washing. After lots of short mummy bye bye's she realised I'd be back. It's all about memory permanence. In her world, if she can't see you right now, you're not there at all. So you need to work on this by playing memory games. Take a favourite toy, put it under a clear container and ask/ show "where's the toy?" oh! It's still here. Then put the same toy under an opaque container and repeat, then under a coloured container. If this game is working, she'll try to look under the container rather then losing interest because her toy is gone.

DO leave her with your mum or somebody else familiar, start with 2 or 3 minutes then come back. Increase the length of time you are gone gradually, and even though there will be tears that you're going, she will soon get over it once she understands that mummy will be back.