I am 21 weeks pregnant & ate two pieces of watermelon that expired yesterday & the water was slimy. Will my baby be okay? Will I get food poisoning?

Baby is insulated. The fetus is protected from many of your health issues by the placental barrier and bag of waters. Only illnesses that have a blood born phase like a flu, measles, blood poisoning could gain access to baby through the blood entering the placenta. Illnesses you might get like food poisoning, that remain within the gut, or other regional areas like a sinus infection, will not get to baby.