It's more than likely nothing. I have had twitching in my biceps before, especially after a week of strenuous workouts. If you can do the dips and pushups and continue to workout normally, you're okay. Some early signs of ALS include slurred speach, stiffness of muscles, and trouble walking normally. It seams to me that you only have one of these symptoms and it is probably as a result of your workout. Strenuous workouts can lead to a loss of Magnesium which has been linked to twitching in the muscles. Most Americans are Magnesium deficient in the first place. Try adding more magnesium to your died and just give it some time, you should be okay.
My brother has ALS. The twitching (fascillations) was his first symptom and it was constant. It never stopped. You do not sound like you have ALS.