How can I combine checkbox and short answer into one survey question?

Do you want to have checkbox and for example text box field inside of one field? This can be achieved with Configurable list widget. 

How to Set Up the Configurable List Widget

I have also Inspected your form https://www.jotform.com/62385109028152 and I saw that you have checkbox field and hidden text box fields that will be shown if certain option from checkbox field is selected. Do you maybe want to combine answers from checkbox field and hidden text box fields? 

You can create additional fields on your form that would be hidden and you can use conditional logic to pass values from two fields to that hidden field.

How to Insert Text or Mathematical Calculation into a Field Using Conditional Logic

I have cloned your form and I have set condition for "I eat red meat." to pass that value and value from "How often do you eat red meat?" field to new field that I have added. I have set my condition like this:

Make sure that fields are set A-Z.

You can set conditions like this for all your fields. 

Here is my demo form: https://form.jotform.com/63296927641971 

Feel free to test it and clone it. 

If I did not understood your question correctly, please explain in more details what you are trying to achieve. 

Hope this will help. Let us know if you need further assistance.