If its been tight all day, rather than a tightening that comes and goes I'd say its baby just having a growth spurt.
About once a week or 10 days and my stomach just feels really tight, the skin feels stretched to its limit and then it wears off the next day. I presume it's baby growing as usually the day or two before I've been really hungry lol! Not sure if this is the same as you or not which is why I've tried to explain my sensations. I would ask the midwife at your next appt but it doens't sound like anything to worry about