Heavy weight lifting and losing weight

Hi,
I started stronglift 5x5 8 weeks ago. The first 5 weeks I lost a little but not much. Then I have started gaining, only 1 1/2 pounds but I am eating at or below my calories. I can feel that my strength is improving and my muscle mass is gaining but I would think that eating at or below my calories I should still be losing. Anyone out there that can HELP?