I had the same situation last year. I took undergrad research for a P/F grade that counted towards my science lab pre-req (my school did not offer letter grades for these research courses but did count toward my science major). Although most schools accepted my PI letter, there were schools here and there that did not and explained that they wanted a science class with a letter grade. Luckily, I was able to get another science letter for these schools.
If you received a letter grade for this research class, I would assume you're in the clear for all schools. If you just received a Pass or Fail grade, you're on a slippery slope and you may want to get an additional science letter from someone who has given you a letter grade.
GL.