Contsipation, blood and mucous in stool

In July I went to Australia for a month and was consitpated for 10 days until I ended up taking laxatives as I felt so sick! A few days after the laxatives wore off, the constipation continued only I managed to go but only a small amount came out. Since then I've still had constipation and about a month ago maybe more, I noticed mucous in my stool. Then a week or two after, I noticed blood in it, not a large amount and it was more mixed in with it as was the mucous. I went to the doctor last week and had some blood tests done for IBS and a few other things but they all came back clear. I have been getting very gassy and my stomach is hurting (not surprisingly as I haven't been to the toilet properly in a very long time!). It is a lot worse at the end of the day after I have been eating and I tend to get quite bloated. My doctor gave me lactulose solution which is a very mild laxative which should help to regulate things. She said it would take 2-3 days to start working but so far it has had no effects.
Any Idea what this could be? What should I do?
Oh also, I'm 18 and female. I get stressed quite easily from studying but thats it!

Thanks,

Sam