Help is my bf cheating on me?

Yesterday I was over at his house in bed with him (supposedly making out)....but we weren't actually making out....he didn't even touch me....he was busy texting sm1 called Nikki (spelled as NIkki).....when I told him i saw the name he said it was short for Nicholas.....could NIkki be short for Nicholas? Or is it more likely 2 be a girl?