Not a medical opinion (of course), but I would think its a great time to start. The first few days are the hardest, and if you're already not feeling well, you may not notice the difference...
That being said, be sure to keep yourself hydrated. Thin the mucous by drinking plenty of lemonade and water, and keep your bowels moving. You may want to try hot/cold showers and or dry skin brushing, etc. Anything to keep the elimination organs clear. You will be amazed at how soon you'll be feeling great.