My freckles are getting darker. Could this be something serious?

No. It might be just exposed to sun too much, if you know it's freckles, not moles.
Freckles. Freckles are problem which is noticeable more and more as sunlight damage is incurred on the skin. There are other reasons for freckle darkening such as chronologic age, but the chief one is exposure to sunlight. Avoid the sun is assiduously as possible and they might get lighter.