Is your resin product bending while curing/after demoulding?

The bending can happen of course due to two reasons: 1. Poor quality Silicone mould that warps as heat is generated and chemical curing reaction of epoxy takes place or 2. Resin that isn't curing evenly.

I'm going to focus on number 2. 

Resin undergoes a crosslinking reaction when you mix the base and catalyst. This reaction creates rigid bonds which change the structure of the liquid to a solid. This reaction also generates heat. 

Heat in various parts of the mould can be different due to how heat escapes. For example if you have a bookmark, heat in the center and bottom will be greater than heat on the sides, because there is more insulation so the heat cannot escape. 

This temperature difference is what may be causing your resin to bend. 

What do you do? Switch to a slower curing resin, or keep the temperature of your workspace controlled.

EVERY 10 degree difference in curing Changes your cure time by 50%!!! A resin piece will cure in half the time at 35 degrees than at 25 degrees. but will the crosslinking be even? NO! Only the top surface that's getting the extra heat will cure faster, creating again A BEND. 

So it's important to control your curing conditions! And choose a resin that dosen't give off heat, or cure too fast.

Equal crosslinking can only be achieved through proper materials and proper cure conditions