I am shocked the onc gave you a diet pill. Almost every onc I know or have heard of tells us weight gain is normal during chemo. Dieting at this point probably won't make any difference but you can make some alterations to your diet - for the long term - that might help.
Cut out as much of the simple carbohydrates as you can. Those are bread, pasta, potatoes, crackers, rice. Those are very easy to digest and the body tends to store them as fat. Also watch veggies and fruit that are very very high in sugar such as peas and grapes (the 2 with the most) and corn.
Keep walking, at least 20 minutes fast paced (where you get your heart rate to increase) every day. If it is too cold or yucky outside go to the mall and walk. You can join all us senior citizen types at the mall walking and walking. But - don't walk through the food court
Hand in there. You can do this. Life will improve after the treatments are done. In the meantime be good to yourself and FIGHT LIKE A GIRL!
take GOoD care