@biobook
The first thing that comes to mind is that a change in antibody tests from 279 to 376 is not necessarily a significant increase. What you'd want to know is how much variability is there in antibody levels in general. For example, if antibodies tend to be higher in the morning than in the afternoon, then you could find a seeming "increase" in antibodies by measuring in the afternoon the first time and then in the morning on the second test, 3 months later. Doesn't mean antibodies are getting higher, just that you've looked at a different point in the cycling levels.
I don't know how antibodies normally cycle, but I do remember posting something along these lines a while back, in response to something. Maybe
@TimT can find it? I vaguely remember a graph showing several individuals and measurements of their antibody levels at different times, and it was not a straight line.
The second thought that comes to mind is that it does take a while for maximal antibody response to develop after an infection, so if the first test was soon after getting sick, then the second test might simply be showing the natural continuation of that antibody response. Again, this is a vague memory, but I think I had posted something about this, that it took some time to develop. Can't remember if it was a few weeks, or a few months. Maybe
@TimT can find that too?