One possibility. Other possible issues could be a virus, trauma, local contact allergy, swollen minor salivary gland. Best to see your dentist and let them evaluate.
Many things. Blisters, erosions and sores in the mouth can be caused by many different things. Some are contagious (herpes simplex virus, coxsackie virus) and many are not (canker sores, lichen planus, pemphigus vulgaris). Hot food can certainly cause blisters in the mouth. Have this checked out to find the exact cause as this is important for long term treatment decisions.
Give it time. Thermal burns are usually self-limiting and will heal on their own. Depending on size, location and severity of pain your dentist might prescribe a mediated mouth rinse to help ease the symptoms. This will allow you to eat and drink more comfortably and prevent dehydration.