Sorry cannot be of much help - but if the vet has given he the all clear then all I can think of is that is is due to her age, My weim used to do this when excited and when we had company - she did not come to me until she were 15 weeks and I remember we had this happen for about a month which would have put her at approx the age of your dog now - she just seemed to grow out of it - hopefully the same will have with your JRT
regards
DT