Sucky baby or forever hungry? - feeding problem?

My 2 week old baby girl latches on very well and drinks well for 20-25 mins on one and then I swap to my other for the next feed. Shes a bit of a sleepy baby and tends to fall asleep at the after about 10 mins but carries on sucking and swallowing for 10 mins with breaks of about 5 seconds between each swallow. Once shes finished she often wakes up 15 mins later as if starving hungry and then latches on for 'snacks' of about 5 mins before falling asleep again. Its becoming exhausting as she gets quite distressed - sucking her hands and head ing my neck trying to find a nipple until I give her more. In the ultrasounds she was always sucking her thumb...do you think this is hunger or need to suck?