Thank you for your question. Although without being able to examine you I can not offer you the specific advice on diagnosis and treatment that you need, but I would try to provide you some relevant information about your health concern.
Usually these kind of trembling symptoms could be associated with fatigue, stress, anxiety or a form of panic attack and called as muscle tremor. Other possible causes of tremors could be overactive thyroid, Parkinsonism disease, alcohol use, certain medications, alkalosis, and low blood sugar. Sometimes previous injury may cause pinched nerve and may be a reason of tremor but this possibility remains low. Therefore, it is quite essential to thoroughly assess the all possibilities here with the help of a doctor. Hope this helps.
I think it's a nervous thing like anxiety. When you're tight with somebody and suddenly you are led to believe that they don't like you as much as you like them, it can be very unnerving. Anytime you get real anxious about some situation, your heart rate will go up, you start breathing shallowly, and this can make all this trembling go on. Whenever you get in that kind of situation, do some real deep breathing, it will slow the heart and reduce anxiety. You should also sit down with your partner and find out what they really think of you, for reassurance, so when little fights pop up, which they always do even with the best of friends, you won't lose it so much. I might add that even tho this only happens with this one person, could be you need more protein and need to eat more food in general, plus enough water and regular exercise, so you won't feel so weak, in case weakness and family upset combine to make you shake.
Trust is a big obstacle to overcome with people you care about, and if you don't trust that all is well, then arguments are more frequent and upsetting. Also, self-confidence is an important thing to give yourself, by doing things in your spare time that are rewarding and fun, like artwork, or music, or dancing, you know, creative kinds of things that make you feel good about yourself. So, hope this helps. The fact that you don't have this shaking with anybody else or in any other situation, well, that tells me, anyway, that it's an anxiety about the relationship that is bothering you, so work on that as best you can, and if you cannot overcome it, then could be you need to take a break from them and rethink where you're going with the whole thing.