Most people who have periods will have some that are lighter (or heavier) than what they usually get. Masturbation doesn't have the ability to stop a period, so that's not the cause of the lighter flow. But lots of things--rest, activity, diet, stress, health changes, even a move from one place to the next --can influence how a period is in a given month.
If the cramps and shakiness are a usual part of your period, then you can assume what you're feeling now is part of that pattern. However, if you notice the cramps or shaking continues past when your period does, you may want to check in with a healthcare provider to make sure there isn't another, underlying cause.