No obvious signs to me either, but if the latest mating date was 3rd feb that would give a due date of 29 may, then allowing for a week late I'd rather be safe than sorry, and keep her alone for another week, then instead of putting her in with the sow put her in with the boar, as if it was a heat last week, then you'll be in time to catch her next one.
Agree with Liz, not all sows just cat fight and get over it, I've got 2 that literally hate each other, it never settles and I no longer attempt to put them together! But mostly once a pecking order is established the dust settles.