Try buying one size larger which will usually give you the right not too tight fit.
Stretch fabric is crucial for comfort especially since at different times of the month you may need more ease.
Uncluttered look - flat front, no pockets or embellishments, is a very flattering look and prevents bulk around the hips.
Be careful with low rises on slim pants because you always have to be careful with how your walk or sit. A relative medium rise will give you more flexibility.
Apply a sense of proportion to your look. For example it is difficult to wear fitted on both the top and the bottom. Try soft blouse, easy fit jacket or long sweater with fitted pants. If you choose to where fitted on top and bottom remember that wearing the same color on top and bottom elongates the frame.
Avoid light colors or patterns on the bottom as they tend to add pounds. Unless you are a twig save those beautiful prints for the top or wear menswear type patterns on the bottom.