I am nursing my 3 month old, and I take Claritin/Sudafed when my allergies are out of control. (The box says "ask a health professional before use" if pregnant or breastfeeding.) I do this only when necessary, not on a regular basis, and it doesn't seem to cause any trouble with the little guy. If you can find a nasal spray that works for you, it might be a better option since less medication would be in your bloodstream. Check with your doctor and/or pediatrician first, of course. :)
For a drug-free approach, you could also try a "neti pot" to clean out your nasal passages with a salt water solution. (Make sure to use plain sea salt with no anti-caking agents.) You can order them online, just do a google search on the word.