Standing in cot & then falling and banging head.

Elizabeth has now worked out how to stand up in her cot, bless her. She's looks so happy stood there, but then she's still shaky on her legs so falls backwards and keeps banging her head on the side. She did it this morning with a real whack when I wasn't there & now has a nice red mark on her eye. We've lowered the mattress but she can still just about grab the side, so it's actually more dangerous as she falls more easily. Any tips?! I've put one of those cot bumpers round the side but it doesn't really do much. I guess it's just one of those stages but I'm concerned she's going to have a sore little head after this!

Thanks
Emma