YTA- for multiple reasons

1: she’s clearly stressed after being pregnant for nine months and giving birth six months ago to your son, probably isn’t sleeping well and might be dealing with PPD (?) and instead of helping her out and making her day better you inconvenience her

2: instead of admitting that you hid the food, you repeatedly lie to her while she’s tired and stressed making her feel like she’s losing it

Both of these aren’t great decisions but the main issue IMO is

3: when she gets upset by a stupid prank that made her life more difficult and made her feel like she was going crazy, YOU get upset and ask her what her problem is??

YTA- also FYI saying something was just a joke does not equal an apology