Brown spit up.

Patient: My six week old is formula fed and tonight he spit up brown spit up. The spit up was not really dark but it was brown. About a week ago the nutritionist wanted us to try him on soy. We tried it for 2 days and it was horible. He started spitting up more frequently. He has been back on his normal formula for a week now but he is still spitting up. He does not have a fever and is acting normal. He is also very hard to burp. I am just worried and wondering what the brown spit up is. I was also wondering if gas drops would help the situation.