It does sound like achilles tendonitis to me.
Have a look at your trainers. Does the heel tab rub against the tendon? Do you need anti-pronatory trainers? A good running shoe shop would guide you. Is the trainer flexible & bending in the correct place? Does the trainer absorb shock?
If you have tight or fatigued calf muscles, this will transfer to the tendon.
Be careful with the exercies you choose to do.