Welcome to QuickBooks Community, papillonvintage.
The most possible reason for this is that you've reached the threshold to pay the National Insurance. There are two pay types of National Insurance:
- Class 2 if your profits are £6,365 or more a year
- Class 4 if your profits are £8,632 or more a year
Let's go to your Taxes tab to verify. Also, you'll want to read this guide about Self-employed National Insurance rates. Additionally, you can check this page to learn more: Rates and allowances: National Insurance contributions.
Keep your posts coming if you need more help. Have a great day!