Help! why has my friendly, loving, JRT suddenly started to bite strangers?

by marie
(west mids)

Hi, my jack Russell terrier, Trixie is 9 yrs old now and has always been very loving and obedient and i could not of wished for a better dog. She is my baby and the whole family adore her. She did have a nasty experience 2 years ago when we lost her for 4 days and luckily she was taken to the rspca hospital where we was reunited. The vet there believed she had been kicked by someone when she was missing as she had an injury to her little back leg. Since this she has been timid and scared around strangers, but recently she has started growling, snarling and on a few occasions nearly biting, even children (who shes always adored).The problem is getting worse by the week!! She has been checked by a vet and has been given a full bill of health, she recommended a dog behaviorist but it is going to cost me £300+ which i simply haven't got. I was wandering if you had any suggestions as to how i can help her overcome this? Ive tried everything i can think of, reassuring her, praise, treats, but nothing seems to work. I would appreciate any suggestions you may have. Thank you.