Your Procreate app could be drawing only straight lines if you have the Drawing Assist activated. To turn it off, open Actions by clicking the wrench icon at the top bar and then click Preferences. Navigate Gesture Controls >Assisted Drawing and then deactivate all settings for Assisted Drawing.
Take the deactivation process a step further by navigating to the Drawing Guides screen and turn off Drawing Assist if activated. Now open the Actions Tab, click Edit Drawing Guide and ensure that Drawing Assist is not enabled on the layer.