Having pregnancies that close together is very hard on your body, as you have probably not yet fully recovered from your previous one. Additionally, if you're breastfeeding, being pregnant at the same time will make adequate nutrition -very- difficult, so make sure you are doing your best to eat well. Taking vitamins is a good additional step, but it is no substitute for adequate nutrition from your food. If you can, see a nutritionist for more specific suggestions. Aside from nutritional problems, I don't know of any specific health risks, but as with any pregnancy, you should start getting prenatal care as soon as possible.
Not necessarily. If you were in fairly good health prior to becoming pregnant you should be fine. I got pregnant when my youngest daughter was only 5 months old.
If you were healthy to begin with no need to worry. I found out I was pregnant when my daughter was 6months old & never had any problems with my pregnancy. Actually it was the best pregnancie out of all of them :)