Hi I'm Jim and single dad to Karen my 13 year old daughter. Recently she has been wearing tight jean type shorts which are quite tight on her, a few times I have noticed her standing watching TV and tugging the waist up so the middle seam rides up her crotch, then she flexes her bum a few times, is she masturbating??
Jim, welcome to the world of new-found teen sexuality, however she has probably found other ways to give herself pleasure before discovering the tight jeans method. I know that being a single parent is a great challenge, especially so when they get into their teen years where they come under the influence of peers, media and their own self discovery.
We are all sexual beings and are aware of those feelings way before puberty sets in. If you haven't had a frank discussion about sexuality and the perils of teen sex, now is the time. It is an uncomfortable conversation, especially if your relationship is feeling the strain of teen rebellion as it occurs during this phase of her life.
Try to keep the doors of communication open as much as possible about any subject, but you will find increasing resistance to her being coached/directed/educated as she succumbs to the life of a blossoming young woman.
A woman's whole pelvic region is quite sensitive to touch, so she is going to be easily stimulated by tight clothes in conjunction to visual stimulation of TV programs, etc. which are more explicit in the UK than they are here in the US. Women can have orgasms internally and externally, so she is probably giving herself some pleasure at different levels.
Hang on tight, because it is going to be a wild and bumpy ride for the next 5 years for you and her. Try to keep her engaged with a family female who can be her guide and a source of information and comfort during her teen years.