Contacts make my eye red ?

I've been wearing contacts for years and recently switched to night and day as I always fall asleep in my contacts anyway and wanted it to be less of a problem. They worked perfectly for about a week or two and then suddenly my right eye (which had just recovered from a corneal ulcer before the new contacts use) got really red with no pain or irritation. I took out my contacts and the eye went almost back to normal with slightly any red left so I put them back in and by the end of the day my eye was red again. I've done this about 3 times already but every time it returns to red. What is the problem ?