Labour pains?... only at night?

i'm 38 wks 4 days pregnant, and for the last 3 nights i've been waking up with period like cramps at 3am and they'd last for an hour or two. last night i timed them, and i had them every 8 mins regularly from 4am to 5.30am. they won't go away when i walk or change position. then i fell asleep at 6 and woke up later with no pain. this has been happening for the past 2/3 nights. during the day i feel the usual twinges of pain every now and then, but nothing regular. anyone have been through anything like this?