I had the same type of problems after the births of both of my children, and I am a very fit person, and eat tons of veggies and fruits and drink only water or herbal teas. I see a naturopath because I like to go the holistic route. She put me on several supplements:
First, a good WHOLE FOOD multivitamin. (Not necessarily from the store by the WF name, but a vitamin that is made from foods.) I actually stayed on prenatal vitamins for about a year after each birth, and then switched to a regular WF multi.
Second, some supplements that boost the immune system: vitamin C (it's in the multi, but take extra until you're not sick all of the time), elderberry, zinc, and most importantly Beta Glucan and/or echinacea. When you feel something coming on, mega dose on the Beta Glucan or the echinacea for the first two days (double the regular dosage). On the third day, go to the regular dosing instructions. If you're just feeling yucky all of the time, go ahead and start on the Beta Glucan or echinacea and stick with it for 2 to 4 weeks, along with some vit C, and elderberry and/or zinc.
Third, there are alot of good homeopathic remedies at your local health food stores for specific illnesses. Check them out. If you're going to a good health food store, they should be very knowledgable on them. I find the people at Whole Foods to be very well trained.
Fourth, probiotics are great for increasing healthy bacteria in your body. Acidopholus, bifidopholus, etc. They are in yogurt, but not in a high enough quantity to be very effective. Get a pill, powder, or liquid in the refrigerated section of your health food store. Take the pill or use the liquid or powder in a smoothie, applesauce, yogurt, etc.
Good luck! If you need additional help, I recommend using a holistic health provider, such as a N.D.--naturopathic doctor.