Thanks for your query,based on the facts that you have posted it appears that you are suspecting to have venous leak
as a cause of your weak erection.
Venous leak can not be suspected or confirmed without a test called Duplex Ultrasound
Scannng of penis .
Please consult qualified Urologist
for evaluation and do not take medications like Cialis without his consultation.
The pain that you get on sneezing
is likely to be due to congestion of the penis during erection and nothing to be worried about it .