Stress is the feeling of being under too much pressure, to the point where you feel unable to cope. People respond differently to stress in different contexts, so a situation that feels stressful to one person may be not be to someone else.
Freelancers, entrepreneurs and solo workers can open themselves up to all kinds of stress: work outside their comfort zone, unpredictable earnings, and workloads that are not sustainable in the long run.
As a solo worker you should learn what triggers your stress, and take care to develop healthy habits to combat feeling overloaded. Setting boundaries and maintaining a work-life balance is key.