Finances on maternity

[Whyehywhy]

Massive argument between me and partner.
Background - 2 children together. 4 year old daughter and 6 month old son. I’m still on maternity leave.
£20 sitting on counter DP had withdrawn from his bank account. I said we should put in piggy bank for when we need it. DP gives me a funny look and says the money is his.
As far as I knew we were sharing income seeing as I need to stay home on maternity to look after our 6 month old.
I tried to say to DP this made me upset he calls it his money when we are supposed to be sharing it.

AIBU?

I’m open to the comments and opinions and some people have had a good point but if he had specific plans for it then fair enough we could have agreed on that.
but him saying “my money” really grated on me and then ended up us both arguing about who pays for what when of course I pay for everything I can with my limited maternity pay and he also does what he can which is how it should be- shared!