Hey there!
Motivation is a tricky thing- for a lot of people, the subject of "motivation" seems to be of one thing over the other- would rather sit on the couch and eat chips than work hard and pole dance. It's the instant gratification over the long-term pride.
Changing your medication may take a while (up to a month) to settle in your system, and you may feel strangely for that time. I'm not sure how much research or information you have (or that your doctor had given you), but when it comes to medication that affects brain chemicals, they can be difficult to predict fully. Your lack of motivation may pass, or... it may not.
In any case, the difficulty often lies in how we talk to ourselves, and how we feel about what we're doing. If, for example, I feel happier eating chips on the couch than I do when dancing, I'll more likely sit and eat chips. Usually the thought process is somewhere along the lines of "These chips taste so good, they're better than dancing." Or, "I can dance later, I would rather just sit." I often come up with excuses; "I'm tired today," "I've had a rough week, I deserve a break," "I'll dance tomorrow."
Motivation, from what I've seen, is as simple as getting off the couch. Physically moving, getting up, changing your actual circumstances, is the best way to do it. Even when you don't feel like it, and you've got a thousand excuses and reasons, the truth is... you want to!
As my boyfriend always says, "Well, if you want to do it, then do it!"
Sometimes, it really is that easy.
Be gentle with yourself, know that your medication may take time to settle and your brain take time to acclimate to the drugs, but remember that you do WANT to dance. You'll get there :)