Are there any physical reasons not to have children back to back? Like get pregnant when baby is around 6 months?

PG Interval. The interval between pregnancies does have an effect on outcomes but the research is variable. Some studies suggest that the safest interval is 18-24 mo and other studies suggest that more than 3 mo is safe. From my perspective as a fertility specialist, your age matters. If you are older then you may not have time to delay because then you may not get pregnant. Best wishes.
Mother fatigue. Have you had a baby yet? Taking care of just one baby properly is a lot of hard work. Taking care of two, especially if you are nursing, is even more difficult. Spacing them out will let you give more attention to each child, and make it easier when paying for college. Close spaced pregnancies also take a toll on the health of the mother.
Time to recoup. Pregnancy puts quite a strain on the body. From the stretching and pulling of the uterus and abdominal wall muscles to support the baby for 10 months to the physical strain of expelling the baby during labor to the metabolic stress of feeding baby before and after it is born. You should wait at least a year between pregnancies to let your body recoup between.