How to make my staffordshire bull terrior healthy again? PLEASE HELP ME!!?

so my dog is nearly 3 years old in october and she was a very healthy pup, since my parents had her spaded last year she has gain a lot of weight and u havnt been able to walk her much due to her constant pulling where shes got stronger. I took her out today cause i've had enough of her looking so huge so i put up with the pulling and tugging we only walked for about 20 minutes and i brang water with us. since then she hasnt been able to walk properly and she has been quiet all day. My mums putting her on a strict diet now and i dont knnow what to do with her i dont want her getting ill shes happy and playful in the garden but gets ill when i walk her what can i do!