From time to time, no matter how healthfully we eat, we can become run down and catch a virus. A slight cold can be a warning to slow down; recovery is likely to be fairly rapid for someone who is generally in good health, and it is considered good for the immune system to learn to deal with such minor ailments.
Spending too much time in the sun could lead to - for instance - mild sunstroke, migraine, or even cramp. You might have a food intolerance which you are not aware of which could cause various symptoms. Rather than rushing to your GP for every minor ailment, it is worth looking at what you eat to see if some adjustments to your diet might make a difference.
There are some conditions for which medical aid is vital, such as broken limbs or intensive bleeding that cannot be staunched. There are also some symptoms where it is important to get to hospital as soon as possible - if you have sharp stomach or chest pains, for instance. If a baby or elderly person is sick it is always wise to have them checked by a doctor since they are more vulnerable than others.
If symptoms of any kind persist for more than a few days, with no improvement, then it is probably a good idea to talk to a pharmacist or doctor. Good nutrition is important in preventing much illness, and can aid in the healing of many minor maladies, but please trust your instincts and take medical advice if you feel it to be necessary.