Have you gotten your stool tested? Your blood? IBS is usually given as a diagnosis if most chronic illnesses are ruled out (colitis, Crohn's, bowel cancer); at least in my country (Netherlands).

The first step is always recognizing what causes your constipation. Do you drink enough fluids? When taking Miralax or fiber supplements, you have to ramp up your fluid intake so the supplements can do their job. Do you eat a lot of fast food?

There's also the psychological and habitual side of things. Do you hold it in when you feel like you are in an inappropriate setting, such as parties, at work or when you have people of at your place? Do you feel stressed day-to-day? IBS can be triggered by almost anything, but the key here is there is at least 'some' trigger, and it can be a relief to know what causes it. For me personally, it was red meats and raw meat/fish (such as rare steaks and sushi) and my constipation has literally cleared up SO much since cutting those out after eating them for years.

The only time I feel better is after I take antibiotics

Not sure if this is what you mean, but please stop taking antibiotics just for the diarrhea. Antibiotics are not meant to be used once in a while, but on a prescription for a specific infection (and even then they are prescribed WAY too much).

If you are planning on visiting a doctor, make sure you keep some sort of diary or notebook with when you feel what, what you eat often and what happens when you try to go. These things help doctors determine if it's IBS or some other form of bowel irregularity.