Correct me if I am wrong, but an investor's main interest to make money. They want you to succeed. If for some reason they are just milking you for information, remember you aren't under any obligation to tell them all your secrets. If the investor isn't a board member, you don't really have to tell them anything at all.
Don't stress too much about your investors. The only thing you can control is focusing on building a killer company. And you may find that your "competitor" sells a similar product to a different customer base, effectively making them not your competitor.
But, you do "owe" things to an investor - like reports, hard work etc. What you do, however, OWE to yourself is that you protect your idea/product to the fullest.
Thus, you only feed them what they need to know, thus disclose only relevant financial info. If they ask for other info be sure to be brief. Don't disclose the actual details about the tech behind it.
I assume if you know who they invested in, you know the amount and probably the terms, thus assess. More investments > higher risk > higher reward. If those investments were <250k, that's normal and I wouldn't be to worried about it.
Remember that your investor investing in a company that does something similar to yours is not necessarily a bad thing. Competition can be a very good thing. It fosters innovation and creativity, helps keep things moving forward and brings more attention to emerging technologies, to name a few, etc. That investment may be better for your company than your presently realize.
Remember, one boat is a cruise; two boats make a race.
Like many said, investors want to make money.