Imuran causes more nausea after almost two years

When I started Imuran almost two years ago, I couldn't take it coz of nausea. Many fellow Crohnies advised me to take the 3 tablets as one dose before bedtime. I have been doing this for the last 2 years. Now, it's causing me constant nausea to the extent that I wake up from sleep coz of being nauseated. I don't know what happened. Any advice?