Please don't wait on this one, go see your gynae or O&G today, or head to A&E if you feel something is off, because b-HCG levels that are not rising well can mean different things and only a doctor with a scan can assess what is going on properly. On one hand the symptoms you have can be reassuring, but on the other hand the numbers do need a professional to interpret them in full context, so it is really not worth guessing from home on this.