Is there a research honeymoon phase?
Is this sort of phase common in research? Should I be worried about
  it?


It is common. Whether you should be worried about it depends on you. For example, can you imagine enjoying this even if you aren't "in love" with this. Getting a PhD, even in something you love, will be hard. There will be petty, stupid things that still require time (formatting one's thesis...). Does that strike you as a cost you're willing to bear, or do you only really see enjoying things if they're going well, there's a clear trajectory, etc.


  How can I assure myself that this is not a fleeting pursuit? That I
  will still enjoy research in 5 years?


There's no real way to be sure this is true if for no other reason than people change in 5 years. But one way I dealt with it was to switch labs for a semester, to disambiguate if I liked all research or just the particular research I was doing. In my case, I discovered that no, I really enjoyed this thing and was not actually just enamored with the idea of research.