Sanity check here, can I get honest opinions?

I have a child 50/50 with my ex.

She kept him over Mother's day weekend as per our agreement. Mother's day fell in my weekend so I "lost" three nights.

Father's day falls on my normal parenting time, no changes here.

I have requested her a couple of days next month to make up the difference. She refuses because "there is nothing specific in the agreement".

Am I in the wrong here? What do you guys do?