Thank you for asking the query :
Early morning erections are the indication that you have intact testosterone levels and a proper neuro hormonal mechanism. However this can be sometimes painful especially after a surgical procedure. Since the catheter is there only till Monday, I recommend you to try some short term measures.
1. If the catheter is attached to thighs with a tape, please make sure, it is loose and flexible enough to accommodate an erection.
2. Keeping a wet, but clean sterile cloth round the penile shaft while sleeping might help. (Please note this is not evidence based suggestion, but this does seem to work in some patients)
3. Avoid sexual / erotogenic stimuli in general for a week and especially before going to sleep.
Ketoconazole (an anti fungal) drug is used for this purpose ( to prevent unwanted painful erections). If the above measures are not working, you can get a prescription for Ketoconazole from your Doctor. It is usually taken as 400 mg single dose in the night.
Hope this helps. Please feel free to ask follow-up questions if you have any. Thank you.