mommamia22 –
Well, you learn new things each day. Not eating fish and meat is a problem because it is a sakana according to the Gemara. The source for this is the Gemara Pesachim 76b right before the mishnah. Although there are those (most famously the Magen Avraham in Orach Chaim 173:1) who wish to say that it appears that there is no danger in our times, many people aren’t willing to say that it isn’t relevant anymore, and therefore regard it as an issur even in today’s day and age.