From my experience, if you don't eat pasta for awhile (or anything for that matter) then reintroduce it to your diet and you have a reaction, it may not mean you are allergic to it. It could take your body awhile to get used to it again.
I went on Medifast a couple years ago to lose weight which is a pre-packaged meal plan. It worked for me but I didn't eat dairy or bread/pasta for 7 months. On one occassion, I had 2 fajita tortillas and 1/2 cup of strawberry daquiry. I obviously went off plan but it was a big mistake. It was probably the sugar from the daiquiry more than the tortillas but the next day I swear I felt like I had a hangover from hell. This is how it was explained to me....Medifast put my body in a state of keytosis which if I understand it correctly, paleo does too if its strict and when you consume sugar (or bread since it turns into sugar), it throws you out of keytosis and your body has to "detox" the sugar again. This detox period is what causes the headaches (much like when you first began paleo).
On another occassion, when I was transisitoning off of Medifast, I began introducing dairy back into my diet. I had went on a business trip, stayed at a hotel and came back home and began itching intensely all over but there were no bumps or anything. I scratched so hard, I bruised myself. I thought I was going crazy or that I had gotten something from the hotel room. I went to the dermatologist and he said I had dermotographism wihich are basically hives that should last a couple years ( a couple years??!!). I began noticing that when I ate dairy (ice cream in particular) I would double over in pain and then I would begin itching and getting the hives. This reaction has decreased over time but every now and then I will have a breakout when I have certain kinds of ice cream ( I dunno why one ice cream is different than another?) I think it has just taken my body awhile to get used to processing dairy again. I try to stay away from it for the most part though.
So, what I'm trying to say is that it may not be that you are "allergic" to it per se, but you may have an intolerance to it that will take time for your body to get used to or the headaches could be from the "detoxing" period. Or maybe you will decide that it's just not worth it and stay away. Good Luck.