Is my penis size causing lack of her pleasure during sex?

Not necessarily: Ask her what turnd her on. Clitoris is the most important female body part for both pleasurable sensation and to achieve orgasm, and may need stimulation. A very small or very thin penis might not suit her perfectly. She might like you to use your fingers and/or tongue. She should tell you. Hopefully, she hasn't lost interest either in sex or in you. Good luck.

Phallus size: From a female perspective, it's not the size that matters.