Hola Zuzana

If you use the present: debo, then this would have a different meaning, it would mean:

I must have eaten earlier 

as in you're wondering whether you ate earlier or not, that kind of nuance.

But it would not mean "should". 

There's an explanation for that specific use of "deber" in the present + haber + participle here in this other lesson. I hope it clarifies it.

Saludos