I received a IRS change to my 2018 return, that added self employment tax. Why did this get changed? The part time job I received money for was shown on a different line.

If you worked on the side and received income for services performed, that is considered self-employment income and is subject to self-employment tax. If you did not report the income correctly, in most cases the IRS will assess the tax after reviewing your return.

The IRS will assume that all income reported on a 1099-MISC in box 7 is self-employment income subject to self-employment tax. Some income reported in 1099- MISC box 3 may also be self-employment income, depending on the circumstances.  Since you indicated in your question that this was a side job, you are acknowledging that the SE tax is owed, so it seems likely that you filled out your 2018 tax return incorrectly.