Help! Thumb sucking, and not feeding.

Hello all,

My baby is two and a half months old. She started sucking her thumb around 2 weeks ago which I was okay with until she stopped crying in the night and started soothing herself by sucking her thumb. She does not wake up in the night for as long as 8 hours but does suck her thumb to soothe herself. She is breast-fed, mostly expressed milk and sometimes directly from the brrast. I am worried she is not drinking enough as she resorts to her thumb all the time. What can I do to break this habit?

Thank you.