I am in slight pain what shall i do?

Since i bought my treadmill last monday i have used it every day for 40minutes, now last night when i was using it i could feel a pain in my Achilles, pain above the ankle, i have rested it, and the pain is slightly better but should i run tonight on my treadmill or have a day off. The only problem is i feel guilty if i don't use it! i go on it for 40-50 minutes and aim to do around 3.5miles or 4 miles!