Testing is mandatory after every small or big change no matter how tight schedule you have or you just changed a small comment inside the code, you have testing due for the changed code.
There is nothing like trust while developing software, no matter how expert or how senior you are in writing source code, you would have to perform testing for each and every change you did in the code.
Yes, you should not feel unhappy if you or another tester finds a bug in your software source code. Following are the enough reasons to celebrate this important discovery:
Same time, do not criticize any developer in case any bug arises in his/her code because so far at least I do not know any programmer, who can write bug-free source code in the world, second this is one of the reasons we have a separate phase in SDLC (Software Development Life Cycle) which we call post production support (or support & maintenance).