often low in calories. need help

I am on several types of medication and believe that some of them may be working as an appetite suppressant. Because of this I find that there can be a lot of days when I am really low on my calorie intake, especially when I did a work out day.

How can I get enough calories into me on those days when I feel like there's no way I can eat any more? I have tired protein shakes and can hardly drink them. It's way to expensive for me to buy a bunch of different ones to try. I was thinking about getting some of the boost nutritional drinks to help add calories but am not sure how healthy they actually are.

Please help I am at a loss as to what to do.