You seem to be really new to PE seeing that you joined here in November of this year. If you are really new and have never stuck with or done something like the newbie routine here I suggest maybe doing that for a month or two and condition yourself for clamping. The hard string like things doesn’t sound good but if they are actually a little thicker then a normal string then I might start to believe that you were expanding your tunica in certain areas which is a sign of progress in clamping as far as I can tell (another description might be you feel ribs on the sides or somewhere on your shaft). The hard string like things if they are in fact as thin as strings could also possibly be little veins branching out from clamping from the main veins on your shaft. The word hard and veins are not two words you want to here together usually because this might indicate thrombosed veins, but somehow I think in your case if you just rest for like a 3-7 days you will recover enough and your dick will lose some of those strange feelings of the hard string like things. Maybe take a rest break like I said and just do a test to see if that is correct or not? Then if the rest clears that up you will know that if you get the problem again in the future it just takes a little rest (it might even help you gains :) ).
It’s probaly nothing you need to worry about at the moment but just be cautious and ask more questions if you feel you need to know something around here since there are many very knowledgable people here in PE.