hi ive been using a pen cap to stretch because i cant get a finger in, and im not sure if its safe
the pen cap is touching my glans and its really annoying and it kinda hurts and itchy!!
will this be a problem?

It should be safe if there are no rough edges. Leave it in for shorter times such as 1/2 hour at a time, and allow rest in between sessions. Be sure it is clean, and lubricate before insertion.

Is this perhaps one of those ("Biro") pen caps which has a hole in the end in case some child gets it stuck in their windpipe? What shape is the end?

As you may note, we are seriously promoting the use of mouldable plastic to create purpose-shaped objects for stretching with a very smooth, blunt end.