I know that medrol (methylprednisolone) dosepak is a steroid, will it have side affect retaining water or bloating?

Short term: Yes, the Medrol (methylprednisolone) Dosepak contains the steroid prednisone. There could be some retention of salt and water by your body that might lead to some edema but probably not to the extent where you would have bloating. In addition, if you did have some edema it would dissipate very quickly when you stop the dosepak which will only be taken for about a week.

Steroids: Steroids use may cause sodium retention and swellings. In high doses of cortisone patient may develop high blood pressure. Medrol (methylprednisolone) dosepack rarely gives elevated blood pressure.