Calories: more or less?

Hello! I have been on myfitness pal for quite some time now. I have lost and gained and what not, the usual. However, I have been doing more research recently regarding eating more calories. If you would like to share, I'd really appreciate any advice or experience from women who have lost weight eating around 1800-1900 calories per day and exercising/building lean muscle, etc. I do a lot of strength training and a lot of high intensity interval training. I just can't bring my mind to accept that I can eat more calories than 1200-1400 and still lose weight, but I am pretty sure I need to eat more to gain muscle. Thoughts please?