Hello,
If you received schedule K-1 after you filed your tax return, you will have to amend your tax return and include the schedule K-1 in your amended return. You received this schedule because you (or your spouse, if married filing jointly) were either a partner in a partnership, shareholder in a scorp or a benificiary in a estate or a trust. In any case, you will have to include your share of income/loss in your tax return, for that reason you will have to amend your return.