I've read hundreds of threads like this, as I was quite apprehensive about piling on the pounds with pregnancy.
It seems to be not a one size fits all model- some mums put on weight quickly at the start, then weight gain slows down, others don't gain until they are mid term, then pile it all on. And then you get the ones who steadily gain throughout the whole of their pregnancies and others that gain only a few pounds for the entire nine months.
Are you eating healthily or stuffing your face? The best advice I've been given is fill up on lots of good quality, nutritious foods - proteins, vegetables, fruits and wholegrains, then have treats in moderation. I had a few weeks of underreating as I was scared of gaining too much, but I'm listening to my body much more now and making slow but steady gains.
If you can't run anymore (I've not been able to since about 12 weeks due to being really breathless due to anaemia) try keeping active by going for walks or try a pregnancy youtube video.
You may be retaining water, which would account for some weight gain, especially if your intake of carbs and sodium has increased.
Also, maybe TMI, but I found that due to sluggish bowels, I hold onto a lot of weight that is just poop weight! I got diarrhoea and lost a shed load of weight just from one good "clearout" The weight didn't return after making sure that I was properly hydrated, so I guess my bowels were storing nearly 1.5 kg of poop (I wasn't constipated either).
Just for reference, I'm 18+5 and have put on 7lb, but didn't gain for the few weeks that I was undereating, so probably should have put on a little more by now.