When the midwife measures you it is just a rough guide and not completely accurate. The same thing happened to me, I was measuring 3-4 weeks later for the last couple of months with my daughter and second son. They were just big babies
When the midwife measured you, did she seem concerned? Did she check to see if you had dilated at all yet? If she didn't seem worried, try not to sweat it (I know, I know - it's easy for ME to say...I'm not the one ready to pop out a baby :)!!!!),
You may just have a bigger baby. You may have extra amniotic fluid. Your baby may have just had a growth spurt. At 37 weeks you are considered full-term, so it is always a good idea to just be aware of any signs that labor is starting - loss of mucus plug, water breaking, regular cramping/contractions, etc...
Good luck and God Bless