It's still income because you sold stock, no matter what you did with the proceeds. As far as the IRS is concerned, the cash balance of an investment account is just like cash on hand or in a bank.
Is it actually a partnership (including limited partnerships and most LLCs) or S corporation interest? The only time a K-1 is needed is if a partnership or S corporation is involved. If it's regular corporate stock owned & sold by you personally, that's reported on a 1099-B and Schedule D, not a K-1.
