How do theorists obtain summer funding after the beginning of February?
It is difficult for a first-year graduate student in theoretical physics to obtain research funding for the first summer at most universities, as you have already learned. If your department offers courses during the summer, there might be TA positions available, though you presumably have inquired about those by now. Another possibility might be to look for a summer TA in the math department, though they will presumably give preference to their own students for any available positions. Other than that, I can only offer sympathy.