Am I likely to lose any grafts due to masturbation and sex?

There is no link between sexual intercourse or masturbation to the loss of grafted follicles.  The risk is traumatic removal of the grafts by picking at them or a traumatic blow to the head.  Provided that your grafts were placed on your scalp and not in your genital region, it is highly unlikely that masturbation caused graft loss.  The grafts were in solidly after the 6th day so waiting until the 11th day certainly carried no risk. 

Greetings, please see below. We advise to avoid sexual activity for first week after hair transplantation.  The rationale is to avoid increasing blood pressure.  There are no studies supporting this however.  
Best 
Anil Shah MD

Masturbation and sex does not cause hair loss in men.  Male pattern hair loss is a genetic trait.  If you are concerned see a doctor for an examination.

I recommend that my patients restrain from any strenuous physical activity for at least a week after a hair transplant. Provided that you did not hit your head against the headboard during sexual activity or sustain any other trauma you are probably going through the normal phases of post-operative healing. I wouldn't stress out about your sexual activity at this point.  You will just have to be patient for the next year as you wait for your grafts to grow in.  

You can have all of the sex you want at 3 weeks after a hair transplant. I usually tell my patients to go for it in just a few days. No problem