Freelance Consulting while on H1B

Hi all,

I am a software engineer with 10 years of experience and am currently working for a software company in the US. I want to make some extra $$ on the side, so wondering if I can provide freelance consulting to some other firms (not in US) once in a while? They will pay me by check, and they wont send me a 1099. Do you guys think it's possible? Is there anything I should be concerned about?