Ask HN: Can I resign and start a similar product?

I work at a successful (but young) startup in a crowded space. I was an early full-time employee (#4) for 2.5 years, resigned in Oct 2016, but after a break, I started consulting for them full-time again, which I've been doing up until now. As of now, I am a consultant for the company and bill a daily rate.

The product is built for SMBs. I want to start a related product but targeting a niche market and built around only one feature that the company offers today.

Am I allowed to do this?