Masturbation after mummy makeover? Muscle contractions... Is that a problem?

It is unlikely that the muscle contractions you described will have any negative impact on your surgical results.  I would check with your surgeon for advice on beginning sexual relations, as opinions may vary.  All activity, including sexual activity, should be introduced gradually.

Congratulations on having undergone the tummy tuck procedure. Your plastic surgeon will always be your best resource when it comes to specific instructions in regards to returning to activities.  Having said the above, it is unlikely that  the muscle contraction you have experienced thus far would have caused a problem; muscle plications generally are quite secure (a significant amount of force would be necessary for disruption).   Disruption of a muscle plication would likely result in symptoms such as pain, swelling, and/or a visible "bulge".Generally speaking, best  to limit strenuous activity for at least 4 weeks after tummy surgery, assuming you are doing well and there have been no complications. Concerns would include an increase in heart rate and the potential for bleeding. Muscle spasm may be painful. Obviously, significant pressure on the surgical site is not a good idea either. It is not uncommon for patients to report increased swelling that occurs after vigorous exercise (including sexual activity), even several months after tummy tuck surgery. 
Generally, care should be taken during the first two weeks after surgery not to allow for elevation of heart rate, blood pressure, and/or direct trauma/contact to the chest wall. Generally speaking, most my patients are able to return to lower body exercise at the 2 week post operative mark, wait to lift/run for about 4 to 6 weeks after surgery.   
In other words, start slow and "listen to your body" as you gradually progress in intensity/duration of all activities including sex and exercise. Best wishes.