Is there any chances of body swalloing or obesity after donating blood???? or if we not take any precautions after donating???

No. Donating blood is a safe and generous thing to do. Except for a tiny bit of soreness at the donation site, and perhaps some mild fatigue (usually not), it's of no immediate or chronic consequence to you. Good luck.
Body swelling? The only immediate concerns after donating blood are becoming orthostatic due to a loss of blood volume from the donation and becoming lightheaded and passing out when the donor suddenly sits up or stands up. The blood bank takes precautions by giving each donor something to eat and some juice to rehydrate to avoid orthostasis. Swelling or weight gain are not side effects of blood donation.