I'm 25 Years old, since mid January I've had a change in bowel movements. Before, I would have once a day, and it's usually a large easy passing, but after, it became a mild constipation, which I have had mutiple times throughout my life so didn't think much of it. Took some ducolax and thought it would be over. But still ended up being constipation, so I decided to fix my diet, eat more fibre and veggies. Nothing changed and constipation kept going, I ended up being too reliant on ducolax for a bowel movement. Recently, I've been experiencing thin stools, and the past two days, it's become dark (I dunno if this has to do with what I've eaten because I have eaten a pack of oreos that day or deep internal bleeding). It usually doesn't get worse than that. I haven't seen any bright blood in stool, I get some twitching, gurgling and dull pains in left abdomen along with thin stools and constipation but that's usually it. 

I plan to visit the GP in a week or so once I receive my medical card, but I'm concerned that I might be making things worse by waiting so long to get checked. On top of that, in case the GP passes it on as something simple, how to I make them push for more tests and scans? I've read too many stories of illnesses getting worse because of doctors ignoring patients and passing it off as so and so because the odds that it is something series is extremely low. 

Any advice would be greatly appreciated.